Magnetic recording techniques have been widely utilized in various fields including video, audio and computer, because magnetic recording techniques have excellent characteristics that recording media can be used repeatedly, signals can be easily electronized, a system can be constructed in combination with peripheral equipments, and signals can be modified easily which other recording systems do not have.
Therefore, with respect to recording media, it has always been required to further improve recording density, reliability and durability to correspond to the requirements such as the miniaturization of equipments, the improvement Blithe quality of recording/reproduction signals, long term recording, and the increase of recording capacity.
For instance, in audio and video uses, it has been required to realize a digital recording system which can realize the improvement of sound and image qualities, and a magnetic recording medium which can record and reproduce further shorter wavelength signals than in conventional systems and is excellent in reliability and durability even if the relative speed of a head and a medium becomes large for answering to the development of picture recording systems corresponding to high vision TV. Further, in computer use, the development of a high capacity digital recording medium has been desired for the storage of increasing amount of data.
For the realization of high density recording of a coating type magnetic recording medium, a variety of methods have been discussed and suggested from the viewpoints of, e.g., using a magnetic iron or alloy powder comprising iron as a main component in place of conventionally used magnetic iron oxide powders, the improvement of magnetic powders such as the atomization or magnetic powders to heighten the packing density and the orientation property to improve the magnetic characteristics of a magnetic layer, the improvement of the dispersibility of ferromagnetic powders, and the improvement of the surface property of a magnetic layer.
For example, for increasing magnetic characteristics, methods of using ferromagnetic metal powders and hexagonal ferrite powders as ferromagnetic powders are disclosed in JP-A-58-122623 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-61-74137, JP-B-62-49656 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-60-50323, U.S. Pat. Nos. 4,629,653, 4,666,770 and 4,543,198.
JP-A-1-18961 discloses a ferromagnetic powder having a specific surface area of from 30 to 55 m.sup.2 /g, a coercive force of 1,300 Oe or more, and a saturation magnetization of 120 emu/g or more produced from a magnetic metal powder having a long axis length of from 0.05 to 0.2 .mu.m and an axis ratio of from 4 to 8, thus suggests to provide fine metal powders having a small specific surface area. There are disclosed in JP-A-60-11300 and JP-A-60-21307 a method of producing a fine .alpha.-iron hydroxide acicular crystal suitable for a ferromagnetic powder, in particular, a ferromagnetic metal powder, and JP-A-60-21307 discloses the production of a ferromagnetic metal powder having a coercive force (Hc) of from 1,450 to 1,600 Oe, and a saturation magnetization (.sigma..sub.S) of from 142 to 155 emu/g from goethite having a long axis length of from 0.12 to 0.5 .mu.m and an axis ratio of from 6 to 8. JP-A-9-91684 proposes to use a ferromagnetic metal powder having an average long axis length of from 0.05 to 0.12 .mu.m and an acicular ratio of 8 or more in a proportion of 5.0% or less of the entire ferromagnetic metal powder, or to use the foregoing ferromagnetic metal powder of crystallite constituting the ferromagnetic metal powder having an acicular ratio of 4 or more in a proportion of 17.0% or less of the entire ferromagnetic metal powder.
Japanese Patent Application No. 10-167224 suggests to use a ferromagnetic metal powder having an average long axis length of from 30 to 120 nm, an average acicular ratio of from 3.0 to 10.0, and the variation coefficient of an acicular ratio of from 5 to 30%. The atomization of magnetic powders, uniformization of particle sizes, shapes, acicular ratios have been advanced.
Further, a hematite nucleic crystal, iron hydroxide, a monodispersed spindle shape hematite particle wherein a specific ion is used, and an extremely fine ferromagnetic powder obtained by reducing the foregoing hematite particle are disclosed in JP-A-6-340426, JP-A-7-109122 and JP-A-9-227126.
For increasing the dispersibility of a ferromagnetic powder, it is proposed to use various kinds of surfactants (e.g., in JP-A-52-156606, JP-A-53-15803 and JP-A-53-116114), and a variety of reactive coupling agents (in JP-A-49-59608, JP-A-56-58135 and JP-B-62-28489).
JP-A-1-239819 proposes a magnetic powder obtained by coating in order a boron compound, an aluminum compound, or an aluminum compound and a silicon compound on the particle surfaces of a magnetic iron oxide to improve magnetic characteristics and dispersibility of the magnetic powder. Further, JP-A-7-22224 suggests a ferromagnetic metal powder containing 0.05% by weight or less of the elements belonging to Group Ia of the Periodic Table and, if necessary, from 0.1 to 30 atomic % of aluminum based on the total amount of the metal elements, and from 0 1 to 10 atomic % of rare earth elements based on the total amount of the metal elements, and 0.1% by weight or less of residues of the elements belonging to Group IIa of the Periodic Table, to obtain a high density magnetic recording medium excellent in storage stability and magnetic characteristics.
Moreover, for improving the surface property of a magnetic layer, a method of improving surface treating process of a magnetic layer after coating and drying is suggested (e.g., in JP-PA-60-44725).
For achieving high recording density of a magnetic recording medium, making shortwave of signals to be used has been aggressively advanced. When the length of the region to record signals becomes the size comparable with the size of the magnetic (powder) material used, clear magnetization transition state cannot be formed, as a result recording becomes substantially unfeasible. For that reason, it is necessary to develop a magnetic material having sufficiently minute particle size relative to the minimum wavelength used, therefore, the atomization of a magnetic (powder) material has been pointed out for a long time.
In a metal powder for magnetic recording, an aimed coercive force is obtained by making a particle shape acicular and providing shape anisotropy. It is well known in the art that for high density recording, the surface roughness of the medium obtained by atomizing a ferromagnetic metal powder must be made small. However, if a metal powder for magnetic recording is atomized, the acicular ratio thereof reduces and the aimed coercive force cannot be obtained. In recent years, a DVC system of recording video signals by digitization has been suggested and an ME tape of high performance and an MP tape of high performance are used.
For example, the present inventors have proposed a ferromagnetic metal powder suitable for a DVC system and a magnetic recording medium using the same (JP-A-7-326035) . This invention is to provide a magnetic recording medium in which a magnetic layer is restrained to have a coercive force of from 2,000 to 3,000 Oe, a layer thickness of from 0.05 to 0.3 .mu.m and surface roughness of from 1 to 3 nm, and specific reversal magnetization component rate is prescribed.
Alumina, titanium oxide and Cr.sub.2 O.sub.3 are used as abrasives for the purpose of improving magnetic layer strength, hardness, durability, running property and head cleaning suitability, and inorganic powders such as carbon black are used for the purpose of improving electric conductivity, lubricating property and the friction coefficient of a magnetic layer.
For example, JP-A-8-255334 prescribes to include nonmagnetic powders having a Mohs' hardness of 6 or more in a magnetic layer in an amount of from 2 to 15 weight parts per 100 weight parts of the magnetic (powder) material, and discloses that the nonmagnetic powders having a Mohs' hardness of 6 or more are one or more powders selected from the group consisting of .alpha.-Al.sub.2 O.sub.3, .gamma.-Al.sub.2 O.sub.3, .alpha.-Fe.sub.2 O.sub.3, .beta.-SiC, and diamond. However, the relationship between the nonmagnetic powders having a Mohs' hardness of 6 or more and the noise is not disclosed.
Since inorganic powders such as an abrasive cause noises and coarse protrusions on a magnetic layer, fine inorganic powders which are sufficient in a small amount and are excellent in particle size distribution have been demanded. For smoothing surface roughness of a magnetic layer and increasing density, it is desired for inorganic powders to have sufficient strength and abrasive property in a small amount, to improve magnetic layer strength, hardness, and to provide durability, running property and head cleaning suitability.
Further, the development of a system mounting a magneto resistive (MR) head more excellent in high density recording/reproduction characteristics has progressed and is now on the market. A magnetic recording medium suitable for an MR head which shows less turbulence of magnetization, low noise and a good S/N ratio has been desired.